BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff835a2dfc>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
PGD b6393067 PUD b20c6067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 15521 Comm: syz-executor4 Not tainted 4.4.145-g2241aa9 #78
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cd18e000 task.stack: ffff8800aace8000
RIP: 0010:[<ffffffff835a2dfc>]  [<ffffffff835a2dfc>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
RSP: 0018:ffff8800aacefcd8  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8800b7ba4000 RCX: 0000000000000000
RDX: 1ffff10016cd2ab0 RSI: ffffffff835a2dd1 RDI: ffff8800b6695580
RBP: ffff8800aacefcf8 R08: ffffffff853227f0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800b6695400
R13: ffff8800b7ba4008 R14: 0000000000000000 R15: ffffffff82f1eea0
FS:  0000000000e30940(0063) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000000baf5d000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8800b5b25d80 ffff8800b7ba4000 ffff8800b5b26150 ffff8800b5b26058
 ffff8800aacefd20 ffffffff835ac877 ffff8800b5b25d80 ffffffff835ac7a0
 ffff8801d33f19a0 ffff8800aacefd48 ffffffff82f33f1c ffff8800b5b25d80
Call Trace:
 [<ffffffff835ac877>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835ac877>] pppol2tp_session_destruct+0xd7/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82f33f1c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f343df>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f345e0>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835ad64a>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835ad64a>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f1ed76>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f1eeb6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81525155>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81525695>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118e00f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff8100362d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100362d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:253
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:284 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:349
 [<ffffffff838c8675>] int_ret_from_sys_call+0x25/0xa3
Code: 49 8d bc 24 80 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 d0 00 00 00 4d 8b b4 24 80 01 00 00 <f0> 41 ff 8e 80 00 00 00 74 64 e8 65 09 db fd e8 60 09 db fd 4c 
RIP  [<ffffffff835a2dfc>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
 RSP <ffff8800aacefcd8>
CR2: 0000000000000080
---[ end trace 695b84244b33626f ]---